This invention relates to a circularly polarized antenna for ground mobile vehicle communication with a satellite, and more particularly to a microstrip antenna.
Major components of a conventional terrestrial mobile radio system are the base station and the transmitting tower. Simply stated, telephone calls are routed to the mobile users by "broadcasting" the voice channels from the transmitter (or a repeater) over a given radius. The base station serves as the interface between the mobile telephone system and the regular telephone network thus permitting the mobile user to access any telephone in the country. Frequencies used by these systems are typically in the following bands: 30-44 MHz, 152-162 MHz, and 450-460 MHz.
Although conceptually simple from a frequency channel standpoint, such systems suffer from a number of technical difficulties. To begin with, the mobile user must stay in the coverage area (line-of-sight) of the base station or one of its repeaters. A more severe shortcoming, however, is its wasteful use of the precious little spectrum allocated to this service. Note that since the voice channels are broadcast over the entire coverage area, any one channel cannot simultaneously be used by more than one user, thus limiting the size of the market the system can address. To overcome these difficulties, a new concept called a cellular mobile radio system is under development.
The cellular mobile system under development and field experimentation will correct most of the deficiencies associated with conventional systems. In a cellular system, the entire coverage area, for example, a major city and its suburbs, is divided into a number of cells, each with its own base station and frequency band. Cellular systems operate in the 806-890 MHz band. The frequency reuse concept is used to increase the utility of the available frequency spectrum. The total allocated band is divided into 3 sub-bands designated A, B, and C, with each sub-band being assigned to a cell such that no cell is adjacent to a cell of the same sub-band. This insures that the transmissions in one cell do not interfere with the independent transmissions in the adjacent cells. However, through power control and geographical separation, the frequency sub-bands are reused in nonadjacent cells thereby providing efficient use of the available spectrum. In this way, any given frequency channel can be used in hundreds of separate geographic locations unlike the older broad coverage systems.
When a vehicle roams from one geographical cell to another, the mobile unit's frequencies are automatically changed to a new set, compatible with the base station in that cell. The control and interconnection with the wireline network is handled by the mobile telephone switching office (MTSO).
In addition to the frequency reuse aspect, the other major feature of the cellular system is the different cell sizes accommodating varying user population densities. For example, the larger cells might represent less populated suburban areas while the smaller cells represent the more densely populated urban areas. Should the market in any given cell increase, the cell can be further subdivided into smaller cells to accommodate the increased traffic. These smaller cells represent a greater reuse of the available spectrum, this providing more channels for the same coverage region.
A cellular mobile system in the Washington-Baltimore area is operated by American Radio Telephone (ART), for which Motorola is manufacturing the mobile equipment, and another the Advanced Mobile System (AMPS), in the Chicago area, is operated by the Bell System.
As sophisticated as the cellular mobile radio-telephone concept is, the fact remains that such a system may not provide coverage to the nonurban areas of the country. A geostationary satellite, on the other hand, is ideally suited for providing communication to virtually any geographic region, no matter how remote.
Conceptually, the satellite system is analogous to the cellular radiotelephone systems in design and similar in operation. For a geographic service area such as the continguous 48 states, the satellite antenna produces a number of contiguous beams whose circular footprints cover the entire service area. These circular footprints nominally represent the -3 dB contours to the beam patterns. Frequency sub-bands are assigned to each beam as in the cellular case with no adjacent beams assigned to the same sub-band. In the design proposed in a Land Mobile Satellite Service (LMSS) there are 87 such beams covering the 48 contiguous states.
The LMSS system is equivalent to the cellular system in that the beam footprints are equivalent to the cells, the satellite is equivalent to remote repeaters for each cell, and the ground base stations within the beam footprints serve to the same function as base stations in the cellular systems, that is, control and wireline network interconnection.
Communicating through a geostationary satellite requires a mobile antenna which has a radiation pattern of relatively high gain in the direction of the satellite and low gain in the direction of the interfering signals, such as from terrestrial communications systems. One way of achieving this is to have a pencil-beam antenna which tracks the satellite position, either mechanically or electronically. Continuous beam steering for general mobile applications is too voluminous and/or expensive.
Another approach is to use an antenna which has an omnidirectional azimuthal and narrow vertical radiation pattern, with its peak gain at the satellite elevation angle. This type of antenna allows the vehicle to change directions without the need to adjust the beam pointing and still have good gain discrimination between the satellite and other elevation angles.
The LMSS satellite uses a circularly polarized antenna to avoid polarization changes in the EM wave, due to Faraday rotation by the ionosphere. A mobile antenna of matching polarization is desirable, but this is difficult to obtain in a single antenna capable of covering elevations from near the horizon to over 60.degree. above the horizon. A small aperture antenna, such as crossed drooping dipoles or planar microstrip patch radiator may be adequate for elevation angles above 20.degree. but for angles below this a vertical array or a quadrifilar helix would be more satisfactory. The more versatile of these seems to be a vertical array of circularly polarized elements which can readily be phased to steer the beam elevation angle.
In an effort to fulfill the requirements of a land mobile/satellite communication link for the LMSS, an omnidirectional, circularly polarized, microstrip antenna element has been invented for automotive use. This element has a thin profile (approximately .lambda..sub.o /12, where .lambda..sub.o is the freespace RF wavelength) and can be stacked to form a linear array.